Impling a trait on T fails to make it accessable for generic types.

[brendanzab]

Consider the following code:

use std::unstable::intrinsics;

/// Returns the size of a type
pub fn size_of<T>() -> uint {
    TypeInfo::size_of::<T>()
}

/// Returns the size of the type that `val` points to
pub fn size_of_val<T>(val: &T) -> uint {
    val.size_of_val()
}

pub trait TypeInfo {
    pub fn size_of() -> uint;
    pub fn size_of_val(&self) -> uint;
}

impl<T> TypeInfo for T {
    /// The size of the type in bytes.
    pub fn size_of() -> uint {
        unsafe { intrinsics::size_of::<T>() }
    }

    /// Returns the size of the type of `self` in bytes.
    pub fn size_of_val(&self) -> uint {
        TypeInfo::size_of::<T>()
    }
}

fn main() {}

I get the error:

type-info.rs:5:4: 5:26 error: failed to find an implementation of trait TypeInfo for 'a
type-info.rs:5     TypeInfo::size_of::<T>()
                   ^~~~~~~~~~~~~~~~~~~~~~

Commenting out the size_of function, I then get this:

type-info.rs:26:8: 26:30 error: failed to find an implementation of trait TypeInfo for 'a
type-info.rs:26         TypeInfo::size_of::<T>()
                        ^~~~~~~~~~~~~~~~~~~~~~

I would have assumed that seeing as type T affects all types you would be able to use the impled methods even if there was no type constraint specified.

[nikomatsakis]

This is not using associated fns as intended by the current implementation. I think the way to implement this would be:

/// Returns the size of a type
pub fn size_of<T>() -> uint {
    TypeInfo::size_of(None::<T>)
}
trait TypeInfo {
    fn size_of(x: Option<Self>) { ... }
    fn size_of_val(&self) -> uint { ... }
}

[brendanzab]

Ah, that's odd. Because Real::pi::<float>() works. But I'm guessing won't be an issue once we get the improved generic path syntax (see #6894)?

[emberian]

This now compiles and runs correctly. Needstest